I'll add to what Philf said. It may take longer to charge or you may not achieve a full charge. It probably wouldn't cost very much to buy a Nimh rated supply from that well-known auction site. I changed the cell in a Dremel copy that I have, from Nimh to LiIon and bought a cheapo charger. I don't think it cost me more than £10 and I also got a spare cell for that price.

Roy, please exclude me from the 'expert' category. :-) When used in radio control models the nimh were always regarded as slightly inferior to the nicads that they replaced. However the chargers remained the same. The nimh had a higher self-discharge rate IIRC. The chargers only changed with the intro of the more exotic lithium batteries.Pete

The voltage of these battery types is virtually the same. The charger will not see any difference from that point of view. However, if the charger is time-based rather than voltage based You may need to charge the NIMH batteries several times to charge them up to full capacity.

Other than the higher capacity and longer service life the NIMH batteries are a full replacement for old NICD batteries.

Do NOT throw the old NICD batteries to ordinary trash bin - they are problem waste and should be treated as that.